1. Field of the Invention
The invention relates to a fuel reservoir for gaseous fuel in a vehicle, in particular a sorption reservoir.
2. Description of the Prior Art
As an alternative to liquid fuels, gaseous fuels can be used, which differ from fuels that are in liquid form in having a lower energy density. Because of their lower energy density, gaseous fuels in motor vehicles or in buses or utility vehicles for local or long-distance travel are stored in pressure reservoirs. Inside such a pressure reservoir, the pressure level is on the order of magnitude of about 200 bar. The tanks of compressed-gas-powered vehicles are filled at filling stations that have gas pumps equipped especially for filling the tanks of compressed-gas-powered vehicles, which make the gaseous fuel available at a pressure of more than 200 bar. Such gas pumps require an upstream compressor in order to offer this pressure, which involves a considerable expenditure of energy in order to maintain the pressure level of about 10 bar.
From U.S. Pat. No. 6,591,616 B2, an infrastructure for storing hydrogen for a hydrogen-fueled vehicle is known. Hydrogen is carried into a vehicle tank by means of a compressor that at the same time serves as a storage unit. The hydrogen, which is at high pressure, is introduced via a metering valve by means of a hydrogen supply line. Inside the hydrogen tank of the vehicle, the hydrogen is absorbed by an adsorption material, which gives off heat. This heat, in the version in U.S. Pat. No. 6,591,616 B2, is dissipated by water cooling. The heat is transported back to the metering valve via a cooling line. The cooling medium is then earned onward from the metering valve to the compressor of the filling station or to the hydrogen reservoir. The cooling medium gives off its heat inside the compressor. With the version known from U.S. Pat. No. 6,591,616 B2, rapid filling of the tank of a hydrogen-fueled vehicle is made possible at relatively high pressures, and by way of the water cooling, impermissibly high heating up of the hydrogen tank of the vehicle is avoided.
From European Patent Disclosure EP 0 995 944 A2, a method for filling a vehicle tank with hydrogen is known. The hydrogen tank of the vehicle includes a metal hydride, at which the hydrogen is absorbed. The heat that occurs in the hydrogen tank is used to heat a metal hydride material in the supply tank of a cooling station. As the heat transfer medium, water is used, which circulates between the tank of the filling station and the hydrogen tank of the vehicle. The metal hydride, which is provided in the hydrogen vehicle tank and is heated by the absorption of hydrogen, is cooled by means of the water, and the water, which is heated in this way, is pumped to the hydrogen tank of the filling station. Inside the hydrogen tank in the filling station, the metal hydride located there is heated again by the heated water, so that hydrogen is given off, and the water functioning as a circulation medium assumes a lower temperature.
In order to assure a maximum range for a motor vehicle with an acceptable size of tank, for a gaseous fuel in that vehicle, sorption reservoirs based on metal hydrides (chemical adsorption), activated charcoal, zeolites or metal organic frameworks (MOFs) in the context of physical adsorption are used. As explained above, when the tank is filled with a gaseous fuel, its binding energy (desorption) is released as heat and is dissipated. The storage capacity of a tank for gaseous fuel decreases with increasing temperature. Gas cools off upon adiabatic expansion. Depending on the isentropene exponent, the cooling effect is enhanced still further, as for example with a gaseous fuel such as methane, CH4. The work produced upon adiabatic expansion amounts to the following (according to R. W. Pohl: Mechanik, Akustik, Wärmelehre [Mechanics, Acoustics, Thermodynamics], Springer 1959, p. 258):
                              W          mol                =                                            R              ·              T                                      κ              -              1                                ·                      [                          1              -                                                (                                                            p                      2                                                              p                      1                                                        )                                                                      κ                    -                    1                                    κ                                                      ]                                              Equation        ⁢                                  ⁢        1                            W=work        R=gas constant        T=temperature        κ=isentropene exponent        p1=pressure upstream of the throttle restriction (filling station)        p2=pressure downstream of the throttle restriction (tank: p2→p2′)        
In the process of filling the tank, the tank pressure p2 rises from the initial pressure with an empty tank to the final pressure. This means that as the tank pressure rises during the filling, the usable cooling energy drops, as a function of the current tank pressure.
FIG. 1 shows the course of the decrease in the cooling energy from adiabatic expansion at a filling pressure p1 of 200 bar, plotted over the reservoir pressure p in bar. With increasing pressure in the tank, or in other words with a descending pressure gradient, this effect lessens.
In the ideal case, the cooling energy should at least partially compensate for the heat of adsorption A liberated, so that the temperature in the tank for a gaseous fuel remains as constant as possible. The change in temperature is determined by the adsorbed gas quantity n. The temperature that a tank assumes on receiving a gaseous fuel is defined by
                              Δ          ⁢                                          ⁢          T                =                                                            n                ·                Δ                            ⁢                                                          ⁢              E                                                      C                Sp                            ·                              M                Sp                                              =                                    n              ·              A                                                      C                Sp                            ·                              M                Sp                                                                        Equation        ⁢                                  ⁢        2                            n=fuel quantity of the gas put in the tank        CSp=specific heat of the reservoir material        A: sorption enthalpy        MSp=mass of the reservoir        
The change in temperature in the tank during tank filling will now be estimated using CH4. If 30 kg of CH4, corresponding to 1875 mol of CH4, are put in the tank, this is equivalent to a liberated heat of adsorption A of 12.5 kJ/mol. The mass of the reservoir is estimated at 200 kg; the specific heat of the reservoir material CSp is 1.3 kJ/kg/K. The temperature rises to approximately 90° C., beginning at an outset temperature of 25° C.
Since in previous introduced conceptions of tank systems for compressed-gas-powered vehicles, there is a high potential for danger in terms of the compressor complexity and the high pressures to be controlled, this is an overall unsatisfactory situation, since the operation of compressed-gas-powered vehicles offers several advantages, particularly with regard to pollutant emissions. The gaseous fuel forms an especially good mixture with air, and with regard to pollutant emissions, gaseous fuel is distinguished by markedly lower amounts of polycyclic aromatic hydrocarbons, compared with gasoline-powered internal combustion engines. Gaseous fuel is maximally free of lead compounds and sulfur compounds and has very good combustion properties with excellent mixture formation and mixture distribution, which is even more pronounced especially at low temperatures.